A Data Acquisition System for Radiation Measurement

Registration # MT2065

Table of Contents

• Overview

The device described here implements a data acquisition system that is specialized for capturing brief repetitive events (analog or digital) that occur asynchronously. The intended application of the system is for measuring ionizing radiation detected by any of three types of sensors: Geiger-Mueller tubes, a scintillation probe, or a PIN photodiode detector. The system is used to detect and measure radiation, including cosmic rays (and Muons), natural background radiation, and emissions from other radioactive objects, such as an antique Radium-dial wristwatch. However, because of the unique features of the design, the system can also be applied to a variety of other measurement problems.

The user interacts with the system via a combination of a graphical LCD display, buttons, switches, etc. A menu system allows the user to modify various software parameters and to select modes of operation. To operate the system for radiation measurement, the user selects a probe type, adjusts the voltage to the probe, selects certain parameters and settings, then initiates a `sampling session'. When the session is complete, the data is displayed on the LCD.

The heart of the system is a group of Data Acquisition Processors that operate in parallel to provide both high performance and flexibility for various applications. Coupled to the Acquisition Processors is an Analog Module that handles the raw inputs from the signal sources. Overall control of the system and the user interface is implemented with a Microchip “dsPICDEM 1.1 Plus” Development Board. And finally a power supply sub-system produces various voltages, including a high-voltage low-current source needed for the Geiger tubes and scintillation probe.

The implementation makes use of the following Microchip devices:

• DsPIC30F4012 processor (three).

• DsPIC30F6014A processor.

• MCP6S26 PGA (two).

• MCP6022 op-amp (three).

• MCP4011-502 digital potentiometer (two).

• MCP608 op-amp.

• MCP3421 A-to-D.

• 24LC16 EEPROM.

A view of the overall system. Clockwise from lower-left are the dsPICDEM board, the commercial DC power supply, the high-voltage power supply board, a pair of type 5980 Geiger tubes (in a pink foam mounting), and finally the Acquisition Processors and Analog Module stack (mounted on the right end of the dsPICDEM board). The latter “module stack” is really the heart of the system.

1. Printed Circuit Boards

The Data Acquisition Processors and the Analog Module (in the “stack”, see picture above) were constructed as printed-circuit boards (PCBs) using the ExpressPCB “Mini-Board” service. The Mini-Board service requires that the board be of a certain fixed size, and includes a quantity of three identical boards. Since multiple Data Acquisition Processors were needed for this system, the PCB was laid out to include two processor modules and one analog module. The extra modules that were not used in this system can be easily applied to future projects.

One of the bare PCBs, which includes two identical Data Acquisition Processors, and one Analog Module. The large copper area acts as a heat sink for the voltage regulator. The three portions of the board were cut apart using a jeweler's saw.

This composite picture shows the top and bottom of the same pair of boards, with all components installed. The 10-pin connectors were laid out so that the boards could be “stacked” by installing male or female headers on the top or bottom of the board, as appropriate. The horizontal 5-pin header is for ICSP programming. There was only one hardware design flaw that was found after the PCBs were made. This was corrected with a white wire that can be seen passing through a hole on the analog module.

• Functional Description

Below is a block diagram of the system. On the left side are signal sources, showing 3 types of radiation detectors plus a test-signal generator. The right side shows the Data Acquisition Processors, which share the various input and control signals. The center of the diagram shows the analog signal processing and the Supervisor Processor which collects and displays the data.

1. Signal Sources

The system was designed to support three main types of detectors for ionizing radiation (alpha and beta particles, gamma rays, and X-rays). Each type of sensor has advantages and disadvantages when compared to the others. The practical application of each of the sensors is discussed in a subsequent section. A fourth input signal option is a Test Signal Generator, which was used primarily during the early development of the system to easily verify basic functionality.

A Geiger-Mueller tube is a device that produces a pulse for each `particle' of radiation that is detected. The pulse is of a binary nature - the amplitude and width are approximately the same in all cases. Therefore, the Geiger tube pulses do not require any analog processing, other than to limit the amplitude to acceptable levels. In the block diagram above, it can be seen that the Geiger tube signals go directly to the Data Acquisition Processors, bypassing the analog circuitry. The collected data consists of a count rate and/or a total count over a time interval. This system employs a pair of Geiger tubes to enable the detection of `Muons' (this is discussed in more detail later).

A scintillation probe detects and measures radiation using a different principle than a Geiger tube. This type of probe is generally much more sensitive, and produces analog pulses whose amplitudes are proportional to the energy level of the detected particle. The collected data consists of a distribution of pulse amplitudes that were measured over a time interval. If the system is calibrated, this data can be used to determine which radioactive isotope is being measured, since the distribution of energy levels is generally unique to each isotope.

A PIN photodiode can also be used to detect and measure radiation. It can be very small and inexpensive, but it suffers from very poor `detection efficiency'. The diode produces very small pulses that must be pre-amplified in a low-noise environment. This is achieved with an MCP6022 op-amp mounted inside the metal detector casing along with the photodiode.

The final signal source is a Test Signal Generator. It accepts a 5 volt square pulse as an input, and produces a low-amplitude trapezoidal pulse of the opposite polarity. An MCP608 op-amp is used, taking advantage of the relatively low slew rate to produce semi-linear ramp outputs.

This picture shows the scintillation probe used with the system. It is a military surplus DT-590A probe, originally intended to measure X-rays produced by Plutonium-239.

2. Analog Module

This portion of the system accepts inputs from the various signal sources, and pre-conditions them for measurement by the Data Acquisition Processors. Under software control, various parameters can be adjusted, including the input selection, the gain (from X1 to X1024), and a reference voltage setting. When measuring signals that swing in only one direction (such as the pulses from the scintillation probe), the adjustable reference voltage allows the Acquisition Processor to see the maximum possible dynamic range. The main analog output includes a low-pass filter to reduce high-frequency noise in the signal. The other output from the Analog Module produces a rail-to-rail edge that is used to trigger an interrupt in the Data Acquisition Processors. It is essentially the same signal as the main output, but with an additional X20 gain factor applied to it. The trigger level can be set via software control. These features are implemented with a combination of MCP6S26 PGAs, MCP6022 op-amps, and MCP4011 digital potentiometers.

An MCP3421 precision analog-to-digital converter is included on the module to allow it to be used for accurate measurements of slowly-changing signals. This feature improves the flexibility of the overall system when used in other applications.

The Analog Module also includes a 24LC16 serial EEPROM. In an application where a series of analog modules was applied to different situations, the EEPROM allows each module to store relevant configuration data, such as gain settings and input selections. In this way, a common Supervisor processor does not have to know what type of signal source is being measured.

The Analog Module can be seen in the picture in the following sub-section. It is the fourth module from the bottom, directly above the three Data Acquisition Processors.

3. Data Acquisition Processors

The Data Acquisition Processors are responsible for actually measuring the data coming from the detectors via the Analog Module. A unique feature of this design is that there are multiple dsPIC30F4012 processors running in parallel, each running the same software. Each processor receives the same analog output from the Analog Module, in addition to the trigger output. The trigger interrupt causes each processor to begin an A-to-D conversion. However, each processor has a different short time delay introduced by the software. Since the input pulse is being sampled at slightly different times by each processor, it becomes possible to achieve a `sampling burst' that is faster than what each processor is capable of by itself. The Supervisor Processor (described below) gathers the sampled data from each processor, and can reconstruct the features of the pulse signal, since the timing for each acquisition processor is known. With this multiple-processor configuration, many types of data acquisition become possible with appropriate software changes. In the present implementation, the system uses three dsPIC30F4012 Data Acquisition Processors, each running at just under their maximum rated speed (about 118 Mhz). Up to eight processors are easily supported, with only a minor change in the Supervisor software.

The software in the Data Acquisition Processors currently supports two main modes of operation, which are selected via I2C commands from the Supervisor. One mode is for use with the scintillation probe, where the signal consists of short duration pulses (about 2 to 5 uS), with variable amplitude. In this mode, each processor measures 126 individual pulses in a single `session'. After a sampling session is complete, the Supervisor then reads the data from each processor, and determines which of the 3 samples (for each pulse) is the highest amplitude. The pulse amplitudes are then tallied and displayed on the graphical LCD, thus producing an `energy distribution'.

As mentioned above, each processor adds a different time delay before capturing a sample in the `scintillation mode'. There is a switch pack on each module that is used to set a unique I2C address. The lower 3 bits of the I2C address are also used to set the time delay. So, the lowest-address processor has the minimum possible delay, and the other two processors each have proportionately longer delays. The delay resolution is about 0.31 microseconds. The Supervisor can send a command to the processors to multiply the minimum delay by a factor. For example, it can tell each processor to use a times-10 delay, which would result in delays of about 0.0 uS, 3.1 uS, and 6.2 uS for the three respective processors.

The second software mode is used with the Geiger tubes, and the processor simply counts the number of events from each of the two tubes, plus the number of counts that are coincident with both tubes (this represents a Muon detection). Since it is not necessary to measure the amplitude of the pulses, only one of the three processors is used in this mode. The Supervisor sends an I2C command to the other two processors to tell them to slow down their CPU clocks and remain idle.

The physical construction of the modules allows them to be `stacked' on top of each other, similar to a PC104 bus system. The connectors that join the modules include common signals such as the analog and interrupt inputs, and the I2C bus. Refer to the schematics for details on these signals. Power is bused to each module via a single-row female header that is visible in the picture below (the black vertical connector with red and black wire loops attached).

Shown above are three Data Acquisition Processors stacked together, such that they can share common input and control signals. The fourth and fifth boards in the stack are the Analog and Interconnect Modules.

4. Interconnect Module

The Interconnect Module mounts on the “stack” directly above the Analog Module (see picture above). The module has two purposes, the first of which is to provide a connection point between the Analog Module signals and the components that are not part of the module stack.

The second function of the Interconnect Module is the Test Signal Generator. In the picture above, it is the small vertical module on the top of the stack. It includes an MCP608 op-amp that is driven by a digital output pin from the Supervisor. The relatively low slew rate of the op-amp causes the output to be a trapezoidal shape with fairly linear edges.

5. Supervisor Processor and User Interface

A single dsPIC30F6014A “supervisor processor” controls the data-acquisition processors, gathers the collected data from each, and formats the data for display. The Supervisor is implemented with a Microchip “dsPICDEM 1.1 Plus Development Board”. The dsPICDEM board includes a graphic display, buttons, and LEDs. The firmware uses these features to implement a menu system that allows the user to adjust various parameters, control the sampling process, and view the data in graphical and text formats.

The main functions of the Supervisor code are to interface with the user, and to send command and control signals to the “module stack”. Control signals include I2C transactions with the Data Acquisition Processors, and SPI and other digital controls to the Analog Module and Test Signal Generator.

An example of one option in the menu. Buttons on the dsPICDEM board are used to select different Gain values. The number in the upper-right is a measurement of the high voltage from the power supply (which is currently off).

A very simple one-level menu is implemented. One button selects a menu option, two buttons are used to change the selected option up or down (for those options that have variable settings), and a fourth button causes the selected option to be executed. At most times, the upper-right corner of the display shows the current high-voltage being measured from the power supply circuits.

The menu options are:

• Scintillator Sample. Initiates a scintillator sampling session using the current settings. The basic steps are: a) Disable the analog signal by setting the gain to zero, b) Command each Acquisition Processor to prepare for interrupts, c) Enable the analog signal, d) Wait 15 seconds, e) Command the processors to stop sampling, f) Read the data from the processors, g) Process and display the data. Some examples of the display are in the “Radiation Measurement” section later in the document.

• Geiger Sample. Initiates a Geiger sampling session. The basic steps are: a) Command the 2^nd and 3^rd processors to become idle, b) Command the 1^st processor to begin counting pulses from the Geiger tubes, c) Wait 7 seconds, d) Command the processor to stop counting pulses, e) Read the pulse counts recorded by the processor, f) Display the total from the 7-second session in the right-most column of the display (shifting other columns to the left), g) Loop back to step b), repeating the loop until a button is pressed. An example of the display is in the “Radiation Measurement” section.

• Scint Defaults. Sets all of the parameters to default values that are appropriate for using the scintillation probe.

• Gain. Adjust the overall gain of the two PGAs in the analog module.

• Channel. Select the input channel of the first PGA.

• Vref. Set the reference voltage used by the PGAs. Settings vary from 0 to 5 volts, in 64 steps.

• Trigger. Set the trigger threshold for the interrupt output. Settings vary from 0 to 5 volts, in 64 steps.

• Delay Factor X 0.31 uS. Command the Acquisition Processors to use a delay factor that will be multiplied by the lower 3 bits of the I2C address.

• Cal. Pulse Width. Set the width of the pulse used to drive the Test Signal Generator.

1. Power Supply

The power supply section of the system consists of two main portions: a commercial linear DC power supply, and a high-voltage generator. A hand-wired board includes a slide switch to control AC power to the entire system, plus a pair of switches that select the polarity and routing of the high voltage needed by the scintillation probe or the Geiger tubes.

A potentiometer is used to manually set the high voltage level. Circuitry is included to sample the HV output via a voltage divider, so that a much smaller proportional voltage can be measured and displayed by the Supervisor processor.

• Detailed Circuit Description

1. Data Acquisition Processors

Referring to the schematic above, the heart of a Data Acquisition Processor is a dsPIC30F4012 MPU (U2). A linear voltage regulator (U1) provides 5 VDC for the MPU. The 5V supply is optionally used as the A/D reference, if R11 is installed.

The MPU clock is derived from a 7.37 Mhz external crystal. Under software control, the crystal frequency can be applied to a 16X PLL, giving an effective CPU clock of about 118 Mhz.

An 8-position surface-mount switch pack is connected to certain input signals, to allow for various modes and settings that are visible to the software. The current software uses switch positions A through C to set the lower bits of the I2C address. Switch positions G and H are used to disconnect the I2C signals during In-Circuit Serial Programming (ICSP).

A pair of 10-pin connector positions allows male or female headers to be installed as needed, so that identical boards can be `stacked' to share certain signals. A 5-pin male header is used for ICSP programming.

2. Analog Module

The Analog Module conditions and amplifies signals that will be input to the Data Acquisition Processors. Several parameters can be controlled by the software, allowing good flexibility in how the module is used.

The schematic above shows three possible signal input sources. Each input is AC coupled, then goes through a resistor voltage divider, and is amplified by two MCP6S26 PGA gain blocks (U3 and U4). By choosing appropriate resistors, the dividers allow each input to be scaled to appropriate levels before being fed to the first PGA stage. By choosing the resistor values appropriately, the dividers also are used to set a fixed input impedance for each channel. Currently, the resistors are chosen to provide a 1 megohm impedance. Note that the dividers do not go to Ground, but instead go to a DC reference voltage (`Vref', which provides an AC ground). The Vref signal also sets the reference for both PGAs, which allows non-symmetrical waveforms to be measured with the greatest possible dynamic range, according to the average voltage of the signal. As an example, the output of the scintillation probe is always short-duration negative pulses, so it is useful to set the Vref to +5V to allow the signal to swing down to 0V. For symmetrical waveforms, it would be best to set Vref at 2.5V, for an equal swing in both directions.

The Vref voltage used by the PGAs is generated by an MCP6022 op-amp in a unity-gain configuration, driven by an MCP4011 digital potentiometer. The op-amp presents a very low source impedance at its output. The potentiometer allows the software to set Vref between 0 and 5 volts, in 64 steps.

Following the second PGA stage is a single-pole low-pass filter, which helps reduce any high-frequency noise in the signal. The output of the filter is the main analog signal that is measured by a D/A channel in the Data Acquisition Processors.

The output from the second PGA is also used to drive another MCP6022 op-amp in a comparator configuration. The comparator threshold can be adjusted by the software via an MCP4011 potentiometer. The processors typically use this signal as an interrupt input, to trigger the start of an A/D conversion.

The PGAs are controlled by an SPI bus from the Supervisor Processor, while the potentiometers use simple digital signals to control their settings.

The Analog Module also includes an MCP3421 18-bit A/D and a 24LC16 serial EEPROM that share a common I2C bus. The differential inputs of the A/D are available on the input connector pins.

3. Supervisor Processor

The Supervisor is implemented with a Microchip dsPICDEM 1.1 Plus Development Board. The board includes many useful features, but the main features used by this system are the graphical LCD display, the buttons, the LEDs, and the dsPIC30F6014A MPU. The board includes a prototyping area which was a handy place to mount the custom “module stack”. Next to the prototyping area is a large female header, which allowed for easy signal connections using individual square pins soldered to wires.

4. Interconnect Module

The Interconnect Module is very simple - it includes primarily only the connection points for the various analog and control signals. However, it also proved to be a convenient place to implement the Test Signal Generator.

An MCP608 op-amp is used for the Test Signal Generator. This device has features that are especially good for applications that require low power consumption with reasonable performance. In this design, it was chosen because it has a relatively low slew rate. When driven by a digital pulse, the output becomes a trapezoidal waveform with semi-linear ramps on the rising and falling edges. The stage is configured with a gain of about 0.034, so that a 5V digital input would produce an output swing of about 170 mV. This small signal allows the Analog Module to be tested at various gain levels.

5. Power Supply

The foundation of the power supply is an old commercial Lambda LXD-A-152 unit which supplies +12 and -12 volts at up to 1 amp. These two voltages are used to generate most other levels needed in the system, except for the dsPICDEM board (which has its own dedicated supply).

U2 is a 5 volt linear regulator whose output provides 5V clamping for signals that go to the MPU circuits (in the schematic, see D8, D9, D14, and D15). The 5V line is also used to produce about 3V via the string of D5, D6, and D7. This voltage is used by the scintillation probe. Note that the output of U2 is not used to power the Data Acquisition Processors, since they each have an on-board regulator.

U1 is an adjustable linear regulator whose output is controlled by potentiometer R2. This variable voltage drives module Z1, a cold-cathode fluorescent lamp converter (CCFL). In its normal use, the CCFL module drives a gas tube at perhaps 200 to 300 volts AC. But when the output of the module is very lightly loaded, the voltage can go much higher. The module's output is increased still more with a voltage doubler circuit consisting of C1, D1/D2, C2, and D3/D4. The small capacitors across the diodes help to balance the diode pairs so that they can safely operate above 1 kV, which is the rating of the individual 1N4007 diodes. The output of the doubler is filtered by R4, C3, R5, and C4, to remove high-frequency noise from the CCFL module.

Switches SW2a and SW3a allow the high voltage polarity to be reversed, since the scintillation probe requires a negative voltage, while the Geiger tubes require a positive voltage. Resistor divider R6 and R7 help provide a means to measure the voltage output by dividing it by a factor of 331, so that it is small enough to be measured by an A/D input on the Supervisor Processor board. Op-amp U3 is a unity-gain buffer for the relatively high source impedance of divider R6/R7. Switch SW3b selects the correct polarity for measurement, while diodes D8 and D9 ensure that no incorrect voltage levels can damage the A/D input. Finally, the measured voltage is routed to the AN0 pin on the dsPIC30F6014A.

On the right-hand side of the schematic, it can be seen that the scintillation probe requires +3V, -12V, and a negative high voltage between about -650 and -950 volts. The bottom portion of the power supply schematic includes the Geiger-Mueller tubes and associated circuitry, which is discussed in the next sub-section.

Above is a view of the power supply board. The module in the upper-left is a CCFL converter from an old computer scanner. The red knob on the left adjusts the high voltage output. The second knob is part of an earlier prototype feature that is no longer used. The slide switches control AC power, and select the correct voltages for either the scintillation probe or the Geiger tubes.

6. Geiger-Mueller Detectors

Please refer to the Power Supply schematic in the previous sub-section. The system uses a pair of type 5980 Geiger-Mueller tubes driven by a high positive voltage through resistor R17 (referring to tube V1 only, for this discussion). The tube normally presents a very high resistance, because the voltage is not high enough to ionize the gas mixture in the tube. But when a particle of ionizing radiation of sufficient energy passes through the tube, an ionization trail is briefly formed, and the tube passes a current for a short time. This results in a negative pulse via C15 and R18, which is clamped by D14. The pulse is routed to an interrupt input on a Data Acquisition Processor, where the software counts the event.

There are many types of Geiger tubes that can be used in the system. The type 5980 is a common military surplus type that is very inexpensive and physically small. However, it is not very sensitive, and so is only suitable for detecting high-energy radiation such as Gamma rays. But this makes it quite effective for measuring rates of cosmic radiation over long periods of time. Other tubes are readily available that will detect Alpha and Beta particles as well. Modern-made commercial tubes are available, although they are more expensive than a typical military surplus tube.

1. Muon Detection

The design of this system includes a pair of Geiger tubes. Referring to the image in the Overview section, it can be seen that the two tubes are mounted with one directly above the other. This allows the detection of Muons, which are a type of high-energy cosmic ray. A `normal' cosmic ray does not have enough energy to excite both Geiger tubes. But a Muon passing through both tubes will trigger a pulse from both of them at the same instant. When the Data Acquisition Processor is interrupted by either of the Geiger tube signals, the interrupt service routine checks to see if both of the signals are true at the same time. If they are, it assumes that the cause was a Muon, and this fact is indicated in the displayed data. The two tubes are mounted vertically above one another (rather than some other orientation) because Muons typically arrive via the upper atmosphere.

7. Scintillation Probe

The scintillation probe contains three main components: a scintillation crystal, a photo-multiplier tube (PMT), and a pre-amplifier circuit, all mounted in a light-proof container. A picture of the scintillation probe is in the `Signal Sources' section of this document.

The scintillation crystal is a piece of material that has florescent properties - when a particle (or ray) passes through the material, there is an interaction that emits a very small amount of light (photons). Next to the crystal is a PMT, which is a device that can detect extremely small amounts of light energy, and convert the energy to an electric current. The PMT requires a very high voltage to operate; up to 950 volts or more, but the current requirement is only a few microamps. Connected to the PMT is a 3-transistor pre-amplifier circuit, which outputs negative pulses whose amplitudes are proportional to the energy level of the particle that excited the crystal.

The amplitude of the output pulses is up to 12 volts (the supply voltage of the probe). The pulse amplitude is related not only to the energy of the excitation particle, but also to the level of the high voltage applied to the PMT. So to measure some types of radiation, it is necessary to change the high voltage level to control the sensitivity. The DT-590A probe used in this system was designed to measure X-rays produced by Plutonium-239 with a 950 volt supply, but at lower voltages it is suitable for detecting higher-energy radiation also. Complete technical details of the DT-590A/PDR-56F probe can be found on the Internet.

8. Photodiode Detector

The third type of radiation sensor used with this system is based on a PIN photodiode. When a high-energy particle interacts with the semiconductor material in the diode, a very small current is produced. The MCP6022 op-amp was suitable as a preamp because of its low input current and low noise. The maximum signal is produced when the diode is reverse-biased with a relatively high voltage (in the 20 to 30 volt range). Since the preamp has a very high input impedance, and the signals are extremely small, the components must be very close to each other and well shielded. For experimentation, the circuit shown above was constructed in a machined aluminum box that was originally made for satellite TV equipment.

This probe was an interesting experiment, but was found to be very inefficient as a radiation detector. This is because the junction in the photodiode has a very small surface area and density. So even if a Gamma ray passes directly through it, there is a good chance that it will not interact with the junction sufficiently. There are specialized PIN photodiodes available from makers such as Hamamatsu that are intended for radiation measurement, but they are rather expensive for hobbyist experiments.

The experimental photodiode probe. In the center of the picture is a small module with the MCP6022 op-amp. Below the module (not visible) is the 78L05 regulator. The PIN photodiode is the dark object just above the module, next to a hole in the aluminum box. A piece of black tape shields the photodiode from ambient light. The pre-amplified output signal is available at the `N' connector. A typical output pulse has an amplitude of 10 to 20 millivolts.

• Radiation Measurement

This section shows some images of actual data collected with the system.

This example display was produced with the Geiger tubes, over an interval of several minutes. This is showing `natural radiation' only - there was no radioactive source near the tubes. The X-axis is time, the Y-axis is the number of counts in each 7-second interval. The majority of the counts are from cosmic rays. Note that the display is split into an upper and lower section. The lower graph shows overall counts from the two tubes. The upper graph shows Muon counts, produced by a particle with enough energy to excite both tubes at the same time. Physicists estimate that Muons reach the Earth's surface at a rate of about 200 per square meter per second. Considering the rather small surface area of the two tubes `in series', this display seems reasonable.

Shown here is data from a 15-second `session' with the scintillation probe. This is only `background radiation'. Since the scintillation probe is so sensitive, some of the activity is probably due to things like Radon gas. The X-axis represents energy levels, and the Y-axis represents the number of events that were detected for those energy levels. The number in the upper-right is the voltage currently applied to the probe. The large peak about of the distance from the right side is perhaps a by-product of cosmic radiation. The long bar on the far right represents data points that were not filled during the sampling session (Because the pulses are negative, the absence of a pulse is interpreted as the `highest energy level' - this is actually a defect in the software which I should fix.)

This interesting display shows emissions from the isotope Americium-241, which emits primarily Alpha particles. Note that most of the energy is concentrated around a relatively low level.

This measurement was made with an antique wrist watch that has a Radium dial (they were common in the U.S. until about 50 years ago). Radium emits particles/rays at a variety of energy levels, as can be seen in the display. The voltage was reduced for this measurement because many of the Radium emissions are relatively high energy, and tend to go “off the scale” when the voltage is too high.

• Ideas For Enhancement

Listed here are some ideas to enhance the system.

• Add a menu option for setting the sampling session timing.

• Implement a `bootloader' in the Data Acquisition Processors, so that the Supervisor could automatically load a new code image into them as needed for different applications.

• Change the Supervisor code to dynamically support a different number of Data Acquisition Processors (it is currently hard-coded for three).

• Replace the dsPICDEM board with a custom board that is smaller and less expensive.

• In the power supply circuit, use a digital potentiometer to control the high voltage output level via the software. This would allow the Supervisor to automatically run complex experiments that required changing the voltage level, especially with the scintillation probe.

• In the Geiger Counter mode, implement an option to do long-term data collection, and log the data to a serial port, or to a flash card, etc.

• In the scintillator mode, implement code that attempts to identify an isotope based on the energy distribution. This would require some `calibration' against known radiation sources. The software could adjust voltage and gain levels to normalize the session data.

---